1. Field of the Invention
The invention relates generally to the field of radio frequency transmissions. The invention relates more particularly to methods and systems for determining signal coverage of in-band on-channel digital audio broadcast transmissions. More specifically, the invention relates to methods and systems for accurately predicting the coverage of such signals using a computer implemented prediction model and modeling system.
2. Description of the Related Art
On Oct. 11, 2002, In-Band On-Channel Digital Audio Broadcasting (“IBOC DAB”) was selected by the U.S. Federal Communications Commission (“FCC”) as the technology enabling AM and FM radio broadcast stations to commence digital operations. The FCC announced notification procedures that allowed operating AM and FM radio stations to begin digital transmissions immediately on an interim basis using the IBOC system developed by iBiquity Digital Corporation (“iBiquity”). Since the adoption of IBOC DAB by the FCC, more than 1800 radio stations in the U.S. have opted to begin transmitting digitally, in addition to their regular analog broadcast mode. Further details of IBOC DAB (and radio broadcast in general) can be found in The IBOC Handbook—Understanding HD Radio Technology, by David P. Maxson, ISBN: 978-0-240-80844-4, Focal Press, 2007, HD Radio Implementation: The Field Guide for Facility Conversion, Thomas R. Ray, III, ISBN 0240810023, 978-0-240-810027, Focal Press, 2008, and National Association of Broadcasters Engineering Handbook, 10th Edition, Edmund A. Williams Editor-in-Chief, ISBN: 978-0-240-807515, Elsevier Inc., 2008, all of which are hereby incorporated by reference in their entireties.
In keeping with the FCC's goal of a common technical standard, on Apr. 16, 2005, the Digital Audio Broadcast Subcommittee of the National Radio Systems Committee (NRSC) approved NRSC-5 as a broadcast standard. There NRSC-5 broadcast standard is hereby incorporated by reference in its entirety. On May 18, 2005, the Consumer Electronics Association and the National Association of Broadcasters, co-sponsors of the NRSC, submitted NRSC-5 to the FCC for public consideration in MM Docket No. 99-325.
The mechanism for potential signal interference is derived from the technique by which IBOC DAB is transmitted. The following excerpt from the FCC's First Report and Order describes the transmission technique: “The iBiquity IBOC systems tested by the NRSC are “hybrids” in that they permit the transmission of both the analog and digital signals within the spectral emission mask of a single AM or FM channel. In the hybrid mode, the iBiquity system places digital information on frequencies immediately adjacent to the analog signal. The digital signals are transmitted using orthogonal frequency division multiplexing (OFDM).”
Broadcast stations must be separated in distance according to the relative spacing between their allocated frequencies. For spectral efficiency, frequencies are reused many times across the country, with the result that stations on the same channel and first-, second- and third-adjacent channels are separated at specific minimum distances. The minimum separation distances were determined by the FCC decades ago, based on estimates of signal interference ratios required by analog radios and predictions of radio signal propagation. Although, the NRSC-5 standard describes the principles on which IBOC DAB is to be encoded and transmitted, it does not describe minimum requirements for IBOC reception.
The FCC engineered channel-to-channel frequency spacing for the channel widths of analog radio transmission prior to the introduction of IBOC DAB. As can be seen in the simplified spectrum diagram of FIG. 2, standard frequency separations result in overlap between the undesired 1st adjacent channel signal 202 and the IBOC sidebands 204, resulting in a low desired-to-undesired signal ratio for the IBOC signal. Lesser degrees of overlap may occur as well, as with the upper 1st adjacent channel signal 208 to the upper desired channel sidebands 206, which result in a higher desired-to-undesired ratio. Undesired signals may appear on only one side of the desired channel, on both upper 208 and lower channels 202, and in various combinations of signal ratios.
Due to IBOC DAB's transmission of two groups of sidebands, successful reception involves a complex relationship between the signal ratios of the adjacent interfering stations and the desired IBOC station. Accordingly, while predicting simple analog signal coverage is comparatively straightforward, determination of actual usable signal coverage of IBOC DAB has required empirical data based on field measurements. There is a need for the ability to predict, on the basis of input data about signal levels (field strengths) of the desired and undesired channels, whether IBOC DAB will be received or not received, due to signal interference.